This proposal is for a Mentored Patient-Oriented Research Award (K23) for a promising young scientist: Dr. Jason Lillis. There is a critical need for treatment innovation to develop new clinical methods that promote better long-term weight control outcomes. Dr. Lillis has excellent training and skills in Acceptance and Commitment Therapy (ACT), a newer cognitive behavioral treatment approach, which has shown potential for improving long-term weight control outcomes. Dr. Lillis has also shown a commitment to researching obesity treatment; however he has received little formal training in obesity. His commitment to focusing on obesity treatment research led him to his current position as Assistant Professor (Research) at the Weight Control and Diabetes Research Center (WCDRC) at The Miriam Hospital and the Alpert Medical School of Brown University's Department of Psychiatry. The WCDRC is a premiere obesity research center led by Dr. Rena Wing. The goal of the this Mentored Patient-Oriented Research Award (K23) proposal is to allow Dr. Lillis to further develop as an independent investigator studying the use of innovative methods for the treatment of obesity. In order further pursue this program of research it is necessary for Dr. Lills to receive broad, formal training in obesity and obesity treatment. Through this K23 award, Dr. Lillis will establish expertise in obesity and current best practice treatment of obesity, as well s further develop the necessary skills to design and conduct funded outcome research applying expertise in ACT to the area of obesity. To accomplish this, Dr. Lillis will commit 100% effort to his training and this project for each year of the award. Through mentorship under Dr. Wing and an expert co-mentor team, Dr. Lillis will: receive training in the causes and health consequences of obesity, and the roles of physical activity and [nutrition] in weight loss treatment; develop expertise in the behavioral approach to weight loss and the objective assessment of important factors related to weight control; gain additional experience in the design and conduct of randomized clinical trials; develop expertise in statistical methods; develop professionally through manuscript preparation, grant writing, and oral presentations; and continue to learn about and practice the responsible conduct of scientific research. Dr. Lillis' long-term goals are: to continue to establish an innovative program of research on the use of novel methods to improve long-term weight control outcomes, establish an independent research laboratory, and provide training and mentorship to junior scientists. The primary site for this award is the Weight Control and Diabetes Research Center (WCDRC). Dr. Wing is the Director of the WCDRC, which has its own 17,000+ sq. ft. state of the art, fully staffed facility. Dr. Lillis and 10 other faculty members, all of whom are involved in NIH-funded research projects, meet weekly to discuss research ideas, results, grant applications, and manuscripts; and the entire WCDRC staff meet for journal club to review recent papers relevant to obesity research. The WCDRC is part of the Centers for Behavioral and Preventive Medicine (CBPM) at The Miriam Hospital, which has an excellent research infrastructure to support trainee development, including centralized leadership and administration, grant oversight, data management and programming, and peer and external quality review of all research proposals prior to submission. The WCDRC is also a part of the Department of Psychiatry and Human Behavior (DPHB) at the Alpert Medical School of Brown University; which has a strong commitment by senior research faculty to mentor, a long history of training post-graduates in patient-oriented research through the K Award mechanism, and the structure in place so that a Mentored Research Scientist Development Award can be implemented effectively. Obesity is a major public health problem, and improving long-term weight loss outcomes is a difficult challenge for the field. Dr. Lillis conducted a pilot study testing an ACT workshop for weight maintenance, which showed improved maintenance and further weight loss at 3 months. ACT is a new generation behavioral therapy and is designed to increase psychological flexibility; or the ability to develop and persis in behavior consistent with one's values in the presence of unwanted thoughts, feelings, and bodily sensations. This acceptance-based approach is theoretically-driven, and presents an alternative to current traditional CBT approaches normally applied to weight loss treatment. The primary aim of the proposed K23 is to conduct a randomized controlled trial comparing the efficacy of an ACT approach, an empirically supported Self-Regulation approach (based on a traditional behavioral model) and a control group for long-term weight loss. All participants will first participate in an initial 16-week standard behavioral weight loss program. At the end of the program, participants will be randomized to [receive a workshop intervention delivered in 3 sessions across 6 months: ACT, Self- Regulation, or a cooking demonstration control group. Assessments will occur at baseline, 4, 12, 18, and 24 months.] The primary hypothesis is that the ACT condition will have better weight loss at 24 months. This project is significant in that it tests an innovative treatment approach designed to improve long-term weight loss outcomes, one of the biggest challenges to the field of obesity.